If CUHK is proclaiming that 肩 resembles a shoulder, I can't see the resemblance.
商
甲

乙6638
合集14199商
甲

後2.17.13
合集13917商
甲

鐵215.4
合集4786商
甲

甲759
合集35191
The top left of 「肩」 was originally a picture of the shoulder bone (scapula), most easily illustrated with those used in oracle bone divination from oxen:

(Left) Oracle bone from the Shanghai Museum*. (Right) Binary image of this oracle bone given in 《甲骨文合集》32028.
*Credits to Herr Klugbeisser, CC BY-SA 3.0, original photo at https://en.wikipedia.org/wiki/Oracle_bone#/media/File:Orakelknochen.JPG.
Later on, the character became more stylised and abstract. 「肉」 (flesh > body part) was added, perhaps to enhance character recognition of what might otherwise highly resemble 「戶」 or 「耳」.
戰國・楚
簡

乙4.61
新蔡葛陵楚墓竹簡秦
簡

日甲75背
睡虎地秦簡
隸定

The bone-shape was later firmly corrupted into 「戶」.
東漢
隸

武梁祠畫象
楷

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